1. Field of the Invention
The present invention relates to an inverter circuit for driving a load such as a motor, and relates particularly to a resonant inverter circuit comprising a snubber capacitor for performing soft switching.
2. Description of the Related Art
Examples of conventional inverter circuits for driving a load such as a motor include the technology disclosed in U.S. Pat. No. 5,710,698, U.S. Pat. No. 5,642,273 and U.S. Pat. No. 5,047,913. For example, as shown in FIG. 8, in a soft switching inverter according to conventional technology, a motor 1 comprising a three phase induction motor or a DC brushless motor or the like is connected to the soft switching inverter as a load, and comprises, for example, an inverter using IGBT (Insulated Gate Bipolar Transistor) Q1 to Q6 as switching elements.
In the inverter, the IGBT Q1 to Q6 are connected to both sides of a DC power source 3 in a three phase bridge structure comprising a U phase, a V phase and a W phase. Free wheeling diodes (FWD) D1 to D6 are connected between a collector terminal and an emitter terminal of each IGBT for the purpose of circulating the regenerative energy produced by the inductive load of the motor 1 and the electric current energy stored by the inductive load. Furthermore, snubber capacitors C1 to C6 for absorbing the surge voltage applied between the collector terminal and the emitter terminal of the IGBT during turn-on or turn-off are connected between the collector terminal and the emitter terminal of each IGBT.
In addition, the DC power source 3 and a smoothing capacitor C9 are connected to the inverter. Mid-point voltage storage capacitors C7 and C8 for storing a mid-point voltage are connected in series to both sides of the smoothing capacitor C9. An inductance L1 which resonates with the snubber capacitors C1 and C2, and a bi-directional switching unit SU1 for channeling the resonant current via the inductance L1 are connected between the connection point of the mid-point voltage storage capacitors C7 and C8 and the connection point of the snubber capacitors C1 and C2 of the U phase. In a similar manner, an inductance L2 which resonates with the snubber capacitors C3 and C4, and a bi-directional switching unit SU2 for channeling the resonant current via the inductance L2 are connected between the connection point of the mid-point voltage storage capacitors C7 and C8 and the connection point of the snubber capacitors C3 and C4 of the V phase. In addition, an inductance L3 which resonates with the snubber capacitors C5 and C6, and a bi-directional switching unit SU3 for channeling the resonant current via the inductance L3 are connected between the connection point of the mid-point voltage storage capacitors C7 and C8 and the connection point of the snubber capacitors C5 and C6 of the W phase.
A configuration as shown above may also be called an auxiliary resonant commutated arm link type snubber inverter. In such a soft switching inverter, if for example the IGBT Q1 is turned off, and the IGBT Q2 is then turned on after a short delay, the charging current of the snubber capacitor C1 and the discharging current of the snubber capacitor C2 flow through the mid-point voltage storage capacitors C7 and C8 via the inductance L1. At the same time, if the IGBT Q4 and Q6 are turned off, and the IGBT Q3 and Q5 are then turned on after a short delay, the charging current of the snubber capacitors C4 and C6 and the discharging current of the snubber capacitors C3 and C5 are supplied from the mid-point voltage storage capacitors C7 and C8 via the inductance L2 and L3.
By charging and discharging the snubber capacitor according to the resonant current of the snubber capacitor and the inductance in this manner, when the IGBT is turned off and the snubber capacitor is charged, because the rise in the voltage applied to the IGBT is delayed according to the time constant applied by the snubber capacitor, ZVS (Zero Voltage Switching) of the IGBT can be realized. Conversely, if the snubber capacitor is discharged before the IGBT is turned on, a free wheeling diode conducts and the voltage and current applied to the IGBT becomes zero, thereby realizing ZVS (Zero Voltage Switching) and ZCS (Zero Current Switching) of the IGBT. Consequently, the loss which occurs during turn-on and turn-off of the switching elements such as the IGBTs, can be reduced.
Furthermore, FIG. 9 also shows a soft switching inverter according to conventional technology, which may also be called an auxiliary resonant AC link snubber inverter. In a similar manner as in the auxiliary resonant commutated arm link type snubber inverter shown in FIG. 8, a smoothing capacitor C9 and the inverter are connected to both sides of a DC power source 3. In the inverter, the IGBTs Q1 to Q6, to which are connected free wheeling diodes D1 to D6 and snubber capacitors C1 to C6 respectively, are connected in a three phase bridge structure comprising a U phase, a V phase and a W phase. An inductance L4 which resonates with the snubber capacitors C1 and C2, and a bi-directional switching unit SU4 for channeling the resonant current via the inductance L4 are connected between the connection point of the snubber capacitors C1 and C2 of the U phase of the inverter and the connection point of the snubber capacitors C3 and C4 of the V phase of the inverter. Furthermore, an inductance L5 which resonates with the snubber capacitors C3 and C4, and a bi-directional switching unit SU5 for channeling the resonant current via the inductance L5 are connected between the connection point of the snubber capacitors C3 and C4 of the V phase of the inverter and the connection point of the snubber capacitors C5 and C6 of the W phase of the inverter. In addition, an inductance L6 which resonates with the snubber capacitors C5 and C6, and a bi-directional switching unit SU6 for channeling the resonant current via the inductance L6 are connected between the connection point of the snubber capacitors C1 and C2 of the U phase of the inverter and the connection point of the snubber capacitors C5 and C6 of the W phase of the inverter.
The only difference between the auxiliary resonant AC link snubber inverter shown in FIG. 9 and the auxiliary resonant commutated arm link type snubber inverter shown in FIG. 8 is the path of the electric current for charging and discharging the snubber capacitors, and the principles involved in achieving ZVS and ZCS at each of the IGBT switching elements are the same.
In a soft switching inverter according to the above conventional technology, the electric current which flows through the IGBT (the switching elements) and the voltage applied to the IGBT can be controlled by forming a resonant circuit comprising the snubber capacitor and each inductance. Consequently, this is effective in reducing the loss which occurs in the switching elements during turn-on or turn-off.
However, because the core capacity required for the inductance is determined by the peak conducted current, as the controlled load current increases, the weight and capacity of the inductance also increases. Consequently, a problem arises in that a soft switching inverter according to conventional technology, which requires three inductances with an electric current which is at least as large as the load current, cannot be made smaller or lighter due to the increase in weight and capacity required for the inductances.
In consideration of the above circumstances, an object of the present invention is to provide a resonant inverter circuit that can be made lighter in weight and smaller in capacity.
In order to resolve the above problems, a resonant snubber inverter circuit according to the present invention comprises: six main switching elements (such as IGBT Q1 to Q6 of the embodiment) which either conduct or are cutoff by means of switching control, wherein three sets of two main switching elements which comprise each phase of a three phase bridge are connected in the three phase bridge, and each set of the main switching elements is connected in series to both terminals of a power source (such as the DC power source 3 of the embodiment); six free wheeling diodes (such as the free wheeling diodes D1 to D6 of the embodiment) connected in parallel between two terminals of each of the main switching elements; six snubber capacitors (such as the snubber capacitors C1 to C6 of the embodiment) connected in parallel between two terminals of each of the main switching elements; a three phase output terminal for connecting a load (such as the motor 1 of the embodiment), connected respectively to a connection point of the two main switching elements comprising each of the sets; a bridge circuit having six auxiliary switching elements (such as IGBT Q7 to Q12, and the protection diodes D7 to D12 of the embodiment) for causing an electric current to flow in a single direction, wherein three sets of two auxiliary switching elements are connected in a three phase bridge, and connection points common to the two auxiliary switching elements which comprise each set of the auxiliary switching elements are connected respectively to the three phase output terminal; and a resonant inductance (such as the resonant inductance Lr of the embodiment) forming a resonant circuit with the snubber capacitors, connected to an opposite terminal to a terminal connected to the connection point of the auxiliary switching elements.
In a soft switching inverter of the above structure, the charging and discharging of the six snubber capacitors is controlled by the resonant electric current flowing to the single inductance, which forms a resonant circuit with the six snubber capacitors which are connected in parallel to the six main switching elements, and the bridge circuit comprising the six auxiliary switching elements which are connected to the inductance. Consequently, whereas in the conventional technology one inductance is required for each phase producing a total of three inductances, in the present invention this number is reduced to one inductance across the entire circuit, making it possible to perform soft switching with less switching loss in the inverter circuit, and operate the resonant inverter circuit more efficiently. Consequently, the inverter can be made lighter in weight and smaller in capacity.
In a resonant snubber inverter circuit of the present invention, it is preferable that one of the auxiliary switching elements of each set of the auxiliary switching elements includes a unidirectional switching element (such as IGBT Q7, Q9, Q11 of the embodiment) which only conducts electric current in a direction flowing into the connection point of the auxiliary switching elements, and another auxiliary switching element of each set of the auxiliary switching elements includes a unidirectional switching element (such as IGBT Q8, Q10, Q12 of the embodiment) which only conducts electric current in a direction flowing out from the connection point of the auxiliary switching elements.
According to the above construction, using an inductance electric current which flows in one direction, those two snubber capacitors among the total of 6 snubber capacitors which are connected in series to the both sides of the power source are deemed to be one set of snubber capacitors comprising one phase of the three phase inverter, and the charging and discharging current of the snubber capacitors of a phase which flows in the opposite direction is deemed to be the charging and discharging current of snubber capacitors of another phase, and so for all the phase combinations in a circuit formed by a three phase bridge connection, it becomes possible to control the direction of electric current flow through each phase.
In a resonant snubber inverter circuit of the present invention, it is preferable that the unidirectional switching elements be elements having a withstand voltage greater than a power source voltage (such as the power source voltage VB of the embodiment) of the power source, in both forward and backward directions.
According to the above construction, it becomes possible for switching control to be performed on the auxiliary switching elements so that all of the charging and discharging electric current patterns of the snubber capacitors can be produced.
In a resonant snubber inverter circuit of the present invention, it is preferable that the unidirectional switching elements be insulated gate bipolar transistors (such as the IGBT Q7 to Q12 of the embodiment), each auxiliary switching element comprises the insulated gate bipolar transistor and a diode (such as the protection diodes D7 to D12 of the embodiment), and the diode is either one of a diode in which an anode terminal is connected to a collector terminal of the insulated gate bipolar transistor, and a diode in which a cathode terminal is connected to an emitter terminal of the insulated gate bipolar transistor.
According to the above construction, high speed voltage-driven switching becomes possible using the control voltage applied to the control terminal of the IGBT. In addition, the characteristics of the IGBT allow switching in which the saturation voltage between the terminals during conduction is low.
In a resonant snubber inverter circuit of the present invention, it is preferable that the unidirectional switching elements are metal oxide semiconductor field effects transistors, each auxiliary switching element comprises the metal oxide field effects transistor and a diode, and the diode is either one of a diode in which an anode terminal is connected to a drain terminal of the metal oxide field effects transistor, and a diode in which a cathode terminal is connected to a source terminal of the metal oxide field effects transistor.
According to the above construction, the characteristics of the MOSFET enable high speed voltage-driven switching by using the control voltage applied to the control terminal of the MOSFET.
In a resonant snubber inverter circuit of the present invention, it is preferable that the unidirectional switching elements are reverse blocking thyristors (such as the reverse blocking thyristors T1 to T6 of the embodiment).
According to the above construction, the characteristics of the thyristors enable current-driven switching of large currents by using the control current applied to the control terminal of the reverse blocking thyristors.